JOHANNESBURG, SOUTH AFRICA - JULY 02: A youth smokes marijuana in Soweto township on July 2, 2013 in Johannesburg, South Africa. Anti-apartheid icon and Nobel Peace Prize laureate Nelson Mandela has been in hospital for three weeks where his condition has been described as 'critical but stable.' (Photo by Christopher Furlong/Getty Images)A new study finds that longtime marijuana use lowers motivation and reward-seeking behavior as the brain produces less dopamine. (Photo by Christopher Furlong/Getty Images)

WASHINGTON (CBSDC) — A new study finds that longtime marijuana use lowers motivation and reward-seeking behavior as the brain produces less dopamine.

In a small sample test, researchers in the United Kingdom scanned the brains of 19 long-term marijuana smokers along with 19 nonusers of the same age and sex, measuring the distribution of chemicals throughout the brain via positron emission tomography.

Marijuana smokers were found to produce less of the “feel good” chemical dopamine than their counterparts over time.

However, “whether such a syndrome exists is controversial,” Michael Bloomfield, a researcher at the Institute of Clinical Sciences at Imperial College London, told reporters of the study he led. Although dopamine is thought to be associated with reward-seeking behavior, empirical evidence of so-called “amotivational syndrome” is lacking.

Due to the researched fact that increased dopamine production has been linked with psychosis, the researchers expected to find higher levels of dopamine in the cannabis users, but instead, their findings found the opposite to be true.

“It has been assumed that cannabis increases the risk of schizophrenia by inducing the same effects on the dopamine system that we see in schizophrenia, but this hasn’t been studied in active cannabis users until now,” Bloomfield said in a statement. The results tie in with previous addiction research showing that substance abusers have altered dopamine systems.